The present invention relates to a vacuum pump used to exhaust gas from a vacuum chamber of semiconductor-manufacturing equipment.
A vacuum pump for generating a vacuum environment is essential to a CVD apparatus, a dry etching apparatus, a sputtering apparatus and the like to be used in a process for manufacturing semiconductors. The demand for a vacuum pump having improved performance has been growing greater and greater in recent years in correspondence with highly integrated and fine semiconductor-manufacturing processes. The vacuum pump is required to provide a high degree of vacuum, be clean, compact, and easy to perform maintenance on.
With the development of a composite semiconductor-manufacturing process, a so-called multi-chamber system in which each of a plurality of vacuum chambers is separately evacuated is mainly employed in semiconductor-manufacturing equipment. Accordingly, the number of vacuum pumps to be used in semiconductor-manufacturing equipment is increasing.
In order to comply with the demand for the improved evacuating system of the semiconductor-manufacturing equipment, a roughing dry vacuum pump is widely used instead of a conventional oil-sealed rotary vacuum pump so as to obtain cleaner vacuum. But the oil-sealed rotary vacuum pump has the following disadvantages:
(A) A great amount of energy is consumed; PA1 (B) A large amount of noise and vibration is generated; and PA1 (C) The degree of ultimate vacuum pressure is insufficient. PA1 (Process 1) Period of time required to exhaust a large amount of gas inside a vacuum chamber; and PA1 (Process 2) Period of time required to maintain a vacuum pressure which has been attained. PA1 and EQU h.sub.1 .gtoreq.h.sub.2.
Firstly, a detailed description of the above disadvantage (A) is set forth below.
The operation period of time of the vacuum pump used in the semiconductor-manufacturing equipment is divided into the following two processes:
The ratio of the period of time of process 2 to that of process 1 is very large. Since the vacuum pump does not carry out the work of transporting gas in the process 2, no work is done by the vacuum pump in principle. However, the conventional vacuum pump consumes a great amount of power both in processes 1 and 2. Attention is paid to the reason a great amount of power is consumed in the process 2.
Referring to FIG. 4 showing the power consumed by the conventional screw type roughing pump with respect to the gas suction pressure, the following points (1) and (2) are noted.
Point (1): Consumed power is 4.0 KW when the suction pressure is in the vicinity of 10.sup.3 torr, i.e., when the vacuum pump starts exhausting gas of a great weight flow from a vacuum chamber.
Point (2): Consumed power is 3.2 KW when the suction pressure has dropped enough.
The ratio of the consumed power of the point (2) to the point (1) is approximately 80%. Much power which does not contribute to effective operations is wasted by tens or hundreds of dry vacuum pumps operating simultaneously in the semiconductor-manufacturing factory. The reason power is wasted is described below in detail by exemplifying a twin rotor type screw vacuum pump.
As shown in FIG. 28, the conventional twin rotor type screw vacuum pump (screw type with a thread groove) comprises two rotors 600a and 600b accommodated in a casing 602 and rotating in opposite directions with grooves 608a and 608b engaging each other. Gas is drawn from a suction opening 601 and discharged from an exhaust opening 602. The vacuum pump further comprises rotary shafts 603a and 603b integrally connected with the rotors 600a and 600b; ball bearings 605a, 605b and 606a, 606b for supporting the rotary shafts 603a and 603b; and timing gears 607a and 607b for obtaining synchronous rotation of the two rotors 600a and 600b. Normally, in this kind of dry pump, a delivery valve (check valve) is not formed in the exhaust opening 602 so as to reduce fluid resistance in the exhaust.
FIGS. 29A, 29B, and 29C are model views showing each process (N=0 through 4) of the suction, transporting, and exhaust of the above-described pump. The portions indicated by chain lines denote thread grooves 608a and 608b formed on the back surface which cannot be seen from the front. Reference symbols (S) (shown in FIGS. 29A, 29B, 29C, and 28) at the center and both edges denote portions to form sealing lines as a result of the engagement between the thread grooves of the rotors 600a and 600b. Accordingly, in the twin rotor type pump having thread grooves, a fluid-transporting space for transporting fluid from the suction side to the exhaust side is constituted by the sealing line (S), the thread grooves 608a and 608b, and the casing 602. Let it be supposed that the left half of the fluid-transporting space is denoted as n=1 through 5, and the right half thereof is denoted as n'=1 through 5. How the transporting space formed in the rotor 600a of the twin rotor transports fluid is described below with attention paid to the fluid-transporting space in the left half.
First process: N=0 shows the state in which the suction process has just started. Attention is paid to gas drawn from the suction side and accommodated in a groove of n=1 as shown by arrows in FIG. 29A.
Second process: Upon rotation of N=1, gas is moved to a groove of n=2 and enclosed in a space cut off from the suction side. The description of N=2, 3 is omitted herein.
Third process: Upon rotation of N=4, immediately after a part of a groove of n=5 communicates with the discharge side, gas on the high pressure discharge side flows back to the groove of n=5. Then, gas which has flowed into the groove of n=5 is exhausted to the exhaust side again with the progress of the entire process.
As described above, the reason a great power is required although the pressure on the suction side of the dry vacuum pump having no delivery valve (check valve) has reached a sufficiently low degree of vacuum is because the third process is included in the process.
The operation of the screw type vacuum pump to be performed after the rotation of N=4 is described below by replacing the screw type vacuum pump with a close coupled type pump. Referring to FIG. 30, the pump comprises a vacuum chamber 700; a cylinder 701; a fluid-transporting space 702 on the suction side of the pump; a fluid-transporting space 703 on the exhaust side thereof; a piston 704; a piston rod 705; a suction pipe 706; an exhaust pipe 707; an adsorption tower 708 for processing reactive gas; and a factory pipeline 709. The pressure in the fluid-transporting space 702 is sufficiently low and the pressure in the fluid-transporting space 703 is approximately atmospheric pressure (P=1 kg/cm.sup.2). Accordingly, as shown in FIG. 30, in this process, the difference in pressure applied to the front and rear of the piston 704 is as large as approximately .DELTA.P=1 kg/cm.sup.2. The piston 704 is required to move to the right against the pressure difference (external load). In this manner, energy which is not contributed to effective operations is lost. This disadvantage is common to positive displacement type vacuum pumps of although description has been made by way of example to the close coupled type pump.
It is conceivable to provide the vacuum pump with a delivery valve (check valve) as used in a compressor so as to prevent the back flow of gas from the exhaust side to the suction side in the exhaust process. But the following disadvantages arise.
The exhaust pressure of the vacuum pump is lower (close to atmospheric pressure) than that of the compressor and the volume flow rate of the vacuum pump is greater than that of the compressor.
A great exhaust amount (for example, equal to or more than 500 liter/min) is required in the semiconductor-manufacturing equipment. Because of the above disadvantages, it is necessary that the passage area of the delivery valve is sufficiently large when it is opened to the greatest extent. To this end, it is necessary to make the lift (moving amount) of the delivery valve sufficiently large, i.e., the use of a large delivery valve is required. A delivery valve having large lift has, however, a slow response. Thus, it is difficult to compose the delivery valve in conformity to a screw type vacuum pump, claw type vacuum pump or a scroll type vacuum pump. In addition, noise is increased by compound vibration of the delivery valve and fluid even though the delivery valve is provided, as previously described as the disadvantage (B).